The invention relates generally to the field of digital image processing and, more particularly, for compensating an image for light falloff. The light falloff may be a composed of lens and/or flash falloff.
Lenses produce non-uniform exposure at the focal plane when imaging a uniformly lit surface. For instance, the light from a uniformly lit gray wall perpendicular to the optical axis will pass through a lens and form an image that is brightest at the center and dims radially. The intensity of light in the image will form a pattern described by cos4 of the angle between the optical axis, the lens, and the point in the image plane.
In addition, other factors such as vignetting contribute to the lens falloff phenomena. Vignetting is a property that describes the loss of light rays passing through an optical system.
In the traditional optical photographic system, every image captured on a negative contains some component of lens falloff. The lens of an optical printer also introduces lens falloff during the printing process. However, this falloff occurs to the negative of the original scene, and thus has the effect of providing a means of partial falloff compensation.
Digital printers have no such built in falloff compensation. In fact, digital printers are calibrated such that a uniform field is produced by printing an image of uniform code values. Consequently, a need exists for a method of compensating a digital image for the lens falloff that occurred at the time of image capture in a similar manner to the traditional photographic printer.
In general, the level of falloff compensation corrected by the optical printer is less than the amount of falloff that the optics of the capture device cause. For this reason, there is a need to compensate specifically for the falloff that the capture system induces into the captured scene.
Several examples exist in the prior art which teach methods of compensating an image for the lens falloff that occurred at the time of capture. In U.S. Pat. No. 5,461,444, Toyoda et al describe a method of recording camera identification code onto the film upon which the image is also captured. This identification code specifies the lens information (focal length, focus position, and aperture value). During digital processing, the identification code is translated by look-up-table to a required level of correction which is applied to the image.
However, it is not always practical or possible to record such information onto photographic film. Consequently, the need exists to compensate for levels of lens falloff in a captured scene with less knowledge about the camera""s optical system at the time of image capture.
In addition, when a flash is used in order to provide more illumination on a scene, there is often observed in the output image an effect similar to lens falloff. Consequently, there exists a need to create a flash falloff compensation for those scenes where flash falloff degrades image quality.
Consequently, a need exists for overcoming the above-described drawbacks. More specifically, a need exists for applying a lens and flash falloff correction without the need of the knowledge of the specific focal length, aperture, and focus position.
It is the object of the present invention to provide a method directed to overcoming one or more of the problems as set forth above. Briefly summarized, according to one aspect of the present invention, the invention resides in providing a correction of the light falloff without the need to provide specific data about the focal length, the aperture and the focus position of the picture taking device.
The object stated above is accomplished by a method for compensating at least one digital image for light falloff comprising the steps of:
providing a plurality of pixel values for each of at least one digital image;
providing a light falloff compensation function;
determining a light falloff correction parameter;
using the light falloff compensation function and the light falloff correction parameter to generate an individual compensation value for at least one pixel value; and
applying the individual compensation value to at least one pixel value.
It is a further object of the present invention to provide an apparatus which outputs at least one image compensated for light falloff. The method is accomplished by an apparatus for compensating at least one digital image for light falloff, the apparatus comprises: a digitizing device to provide at least one digitized image frame; a light falloff compensator for applying an individual compensation value to at least one pixel element; a light falloff compensation mask is generated by a light falloff compensation function and a light falloff correction parameter to generate the individual compensation values for each pixel element; and a rendering device for providing a visual representation of an image compensated for light falloff.
These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings.